Chip-Scale Atomic Clocks (CSAC) include vapor cells of alkali metals; typically either rubidium (Rb) or cesium (Cs). An optical beam propagates through the vapor, exciting hyperfine transitions in a phenomenon called coherent population trapping (CPT). An exemplary rubidium-based CSAC, for example, works by exciting the D1 hyperfine transition using a vertical cavity surface emitting laser (VCSEL) that is tuned to the broad absorption at 795 nm and is radio frequency (RF) modulated at 3.417 GHz, which is precisely half the D1 transition frequency. In the early days of CSAC development, Cs was preferred over Rb because readily available VCSELs at 852 nm could be used to excite hyperfine transitions in 133 Cs vapors. More recently as 795 nm VCSELs have continued to mature, Rb has been gaining favor. Rubidium has a simpler Zeeman structure, which provides better signal-to-noise ratio than Cs. Rubidium also has a lower vapor pressure than Cs, which allows CSACs to operate at higher temperatures.
CSACs are not simply shrunken versions of bench-top atomic clocks, however. Several attributes that are unique to CSACs dominate the stability, performance in the field, and reliability of the CSAC. One of the critical attributes is stability of optical power transmitted through the vapor cell. Chip-Scale Atomic Clocks (CSACs) require a laser, such as a vertical cavity surface emitting laser (VCSEL), to emit radiation in a very stable wavelength and having a stable output power. If the optical power level varies or if the wavelength varies, the vapor in the vapor cell of the CSAC experiences an AC stark shift that causes the clock frequency of the CSAC to change.
The optical beam emitted from the laser reflects off of several partially reflective surfaces in the CSAC. If any portion of the optical beam emitted by the laser is reflected off of one of the several partially reflective surfaces in the CSAC back into the laser, the wavelength and/or the output power level of the laser is altered due to optical feedback effects. This optical feedback creates both noise and changes in the power of the output optical beam, which translate into noise and changes in the clock frequency of the CSAC.